DEV: add Pixi task descriptions

[j-bowhay]

closes #197

I thought I should get with the times and learn pixi!

➜  array-api-extra git:(gh-197) pixi task list
Tasks that can run on this machine:
-----------------------------------
clean-vendor-compat, clean-vendor-extra, copy-vendor-compat, copy-vendor-extra, coverage, docs, ipython, lint, mypy, open-coverage, open-docs, pre-commit, pre-commit-install, pylint, pyright, tests, tests-ci, tests-cov, tests-vendor

 - clean-vendor-compat Delete the existing vendored version of array-api-compat
 - clean-vendor-extra Delete the existing vendored version of array-api-extra
 - copy-vendor-compat Vendor a clean copy of array-api-compat
 - copy-vendor-extra Vendor a clean copy of array-api-extra
 - coverage        Generate test coverage html report
 - docs            Build docs
 - ipython         Launch ipython
 - lint            Run pre-commit, pylint, mypy, and pyright
 - mypy            Type check with mypy
 - open-coverage   Open test coverage report
 - open-docs       Open the generated docs
 - pre-commit      Run pre-commit
 - pre-commit-install Install pre-commit
 - pylint          Lint using pylint
 - pyright         Type check with basedpyright
 - tests           Run tests
 - tests-ci        Run tests with coverage and vendor tests
 - tests-cov       Run tests with coverage
 - tests-vendor    Check that array-api-extra and array-api-compat can be vendored together

[lucascolley]

thanks Jake, LGTM! Great to see more people gravitating to Pixi :)